Copying machines nowadays are used in combination with various devices including an automatic document feeder and a sheet post-processing device so as to automatically conduct copying and post-processing operations, such as stapling and punching on the sheets on which images are formed. A sheet post-processing device conducts post-processing such as stapling and punching on each set consisting of a predetermined number of sheets ejected out of a copying machine after images are copied thereon.
U.S. Pat. No. 5,407,188 discloses an example of such a sheet post-processing device.
As shown in FIG. 14, as sheets 101 are ejected via ejection rollers 102 onto an ejection tray 103 after undergoing post-processing, a sheet post-processing device 100 sorts sheets 101 by reciprocating the ejection tray 103 horizontally and at a right angle to the ejection direction of the sheets 101.
The ejection tray 103 is formed so that the sheets 101 are ejected diagonally upward. In addition, freely rotatable sheet edge stopper columns 104 for aligning the trailing edges of the sheets 101 stacked on the ejection tray 103 are erected on the trailing side of the ejection tray 103 with respect to the sheet ejection direction.
The ejection tray 103 is lifted and lowered along the sheet edge stopper columns 104 by a lifting and lowering motor (not shown).
With the sheet post-processing device 100, if while the sheets 101 stacked on the ejection tray 103 are in contact with the sheet edge stopper columns 104, the ejection tray 103 reciprocates to sort a newly ejected sheet 101, the sheet edge stopper columns 104, although freely rotatable, act as immovable objects to the light stacked sheets 101. Consequently, the stacked sheets 101 are disturbed by the friction between the sheets 101 and the sheet edge stopper columns 104.
Since the sheets 101 is light in weight, even if the ejection tray 103 moves horizontally, the sheet edge stopper columns 104 exerts frictional forces on the sheets 101 in such a manner to keep the sheets 101 from moving. Here, if the sheet edge stopper columns 104 rotate in response to the horizontal movement of the ejection tray 103, the alignment of the sheets 101 in the stack can be prevented from being disturbed.
As shown in FIG. 15, the sheet post-processing device 100 disclosed in the above-identified patent application includes a sponge rubber 105 provided to the ejection tray 103 so as to be in touch with, and slide on, the sheet edge stopper columns 104, forcing the sheet edge stopper columns 104 to rotate in response to the horizontal movement of the ejection tray 103.
With the sheet post-processing device 100, when the ejection tray 103 moves horizontally, the sheet edge stopper columns 104 are driven to rotate by the movement of the ejection tray 103. However, when the ejection tray 103 moves vertically, the sheet edge stopper columns 104 exert frictional forces on the ejection tray 103 and interrupt the smooth vertical movement of the ejection tray 103. Consequently, the stacked sheets 101 are disturbed out of alignment.